FIG. 17 illustrates a conventional apparatus, disclosed in Japanese Patent Application Laid-Open No. 7-20097 for instance, for detecting the sound pressure level of a hammering or striking sound by means of a sound pressure detector such as a microphone thereby to find an internal defect or flaw based on the sound pressure level thus detected. In FIG. 17, 2001 designates a concrete product, 2003 a hammer device having a hammer head 2002, and 2005 a fast Fourier transform machine connected with a sound level meter 2004 and a display device 2006.
Now, reference will be made to the operation of the above-mentioned prior art. Conventionally, in such a destructive inspection, an inspector hits or strikes a measuring surface lightly with an inspection hammer so as to detect a defect, which may exist in the interior of a concrete structure, by a difference in the tone of a hammering sound generated at that time. This method is an inspection according to human senses, and hence the criterion for determination is not constant, so that variations in the inspection results would be caused due to the inspector's experience, intuition and the level of his or her skill, thus making vague the records of the inspection results.
Moreover, attempts have been made to apply a destructive inspection using supersonic waves, but the frequency band efficiently output by known probes is several hundred kHz or more, so the supersonic waves are liable to be scattered by aggregates in the interior of the concrete  structure because of their wave length, and the range capable of being inspected has been limited to a shallow area from the surface. In addition, upon inspection, it is necessary to grind the measuring surface and coat it with grease or the like so that matching of the acoustic impedances can be effected to permit acoustic waves to be transmitted to the measuring object. As a consequence, a certain period of time is required for preparation of the measurement, thus giving rise to a problem for practicable workability.
As a method of quantitatively evaluating such an inspection by a hammering sound, Japanese Patent Application Laid-Open No. 7-20097 for instance discloses a method of identifying an internal defect with the level of the sound pressure of a hammering sound detected by a sound level meter. In such a conventional inspection apparatus, the concrete product 2001 is hitted or struck lightly with a fixed impulsive force by using the hammer device 2003 equipped with the hammer head 2002. The hammering sound generated at that time is collected by the sound level meter 2004, and converted into an electric signal. The hammering sound thus converted into the electric signal is recorded by the fast Fourier transform machine 2005, and output to the display device 2006.
An impact generated in the concrete product 2001 by striking becomes a vibration on the surface, which vibrates the air at the boundary surface, thereby propagating as a sound. This sound is converted into an electric signal through a sound collecting device such as the sound level meter 2004. In cases where there exists an abnormality with decreased mechanical strength in the interior of the concrete product 2001, the magnitude and frequency of the vibration generated on the surface are different from those in the case of a sound one without any abnormality, and the magnitude and frequency of the sound generated also differ accordingly. As a result, the internal defect can be detected by comparing the vibration frequency and the sound pressure level of the impulsive sound converted into  the electric signal.
With such a conventional method, however, a resonance sound determined by the material and the shape of the hammer is generated from the hammer itself by an external impact and mixed with the striking sound on the measuring surface, so that different striking sounds can be observed at the same location depending upon the kind of the hammer, thus resulting in a failure or error in the abnormality determination. In addition, there has been another problem that the striking sound could not be identified or discriminated correctly if surrounding noise exceeds the level of the striking sound to a remarkable extent, or if the reverberation of the striking is mixed with the striking sound.
Moreover, the vibration generated on the measuring surface can be changed by the striking angle and the shape of the striking surface, and hence the characteristic of the impulsive sound generated might also be changed, thus sometimes causing a wrong diagnosis.
Further, the level of the striking sound detected could be changed greatly depending upon the distance between the point of striking and the sound collecting device, the direction therebetween, etc., so it is necessary to correct the installation position and the direction of the sound collecting device, or alter the settings of the criterion each time there takes place a change in the distance between the sound collecting device and the striking point.
Accordingly, there arises a further problem that in order to quantify the striking sound under a certain fixed criterion or standard to detect an internal abnormality, it is necessary to select the shape of the hammer and make the measurement environment constant.